1. Field of the Invention
The present invention relates to integrated optical waveguide circuits used in optical components for optical communications and optical information processing, more specifically to an integrated optical waveguide comprising an optical power splitter integrated with an arrayed-waveguide grating wavelength multiplexer, and an optical line test system for testing individual optical lines using the integrated optical circuit in a passive double star (PDS) system for branching an optical line using only passive optical components.
2. Description of the Prior Art
Recently, with the advance in an optical communication system and with the aim of expanding application of the optical communication system, various types of optical waveguide circuit components have been developed. In particular, a planar lightwave circuit comprising glass waveguides formed on a silicon substrate is drawing attention as a practical optical component for its reduced optical loss. A typical circuit of this kind is an arrayed-waveguide grating wavelength multiplexer, which is disclosed in H. Takahashi, K. Kato, and I. Nishi, "Arrayed-Waveguide grating for wavelength division multi/demultiplexer with nanometer resolution," Electron. Lett., vol. 26, pp. 87-88, 1990 and A. R. Vellekoop and M. K. Smit, "Four-Channel Integrated-Optic Wavelength Demultiplexer with Weak Polarization Dependence", J. Lightwave Technol. vol. 9, pp. 310-314 (1991). Further, as an example of an optical power splitter, one which uses light diffraction in a slab optical waveguide is disclosed in S. Day, E. Bellerby, G. Cannel, and M. Grant, "Silicon based fiber pigtailed 1.times.16 splitter," Electron. Lett., vol. 28, pp. 920-922, 1992, and an N.times.M splitter is described in C. Dragone et al. "Efficient Multichannel Integrated Optics Star Coupler on Silicon", IEEE Photonic Tech. Lett. vol. 1, pp. 241-243 (1989).
Construction of an arrayed-waveguide grating wavelength multiplexer is shown in FIG. 1. The arrayed-waveguide grating wavelength multiplexer comprises a plurality of input optical waveguides 301 formed on a substrate 1, a slab optical waveguide 401 for demultiplexing the light to the plurality of waveguides, a plurality of arrayed-waveguides (i.e., an array of waveguides) 302 with different lengths disposed after the slab optical waveguide 401, and slab optical waveguides 402 for making interference of light emitted from the arrayed-waveguides 302, and a plurality of output optical waveguides 308.
The arrayed-waveguide grating wavelength multiplexer has a function to demultiplex light input from an optical waveguide according to the wavelength of the light, and output the demultiplexed light to the individual output optical waveguides. Further, it is also possible to multiplex light of a different wavelength by reversing the input side and the output side. Such a function to select the destination of light in accordance with the wavelength of the incident light is referred to as a wavelength routing function. Such a wavelength multiplexing/demultiplexing function is very effective for wavelength multiplexing or routing using the optical wavelength in the optical communication system.
Further, construction of an N.times.M star coupler type optical power splitter is shown in FIG. 2. In the N.times.M star coupler type optical power splitter, light incident from any one of N input optical waveguides 304 is distributed to the individual M output optical waveguides 309 independent of the wavelength. Therefore, this device is important in a system requiring demultiplexing of light to a number of lines. The device comprises a plurality of input optical waveguides 304, slab optical waveguides 406 for developing light confined in the channel waveguide, and output optical waveguides 309 for receiving and outputting the developed light.
In short, the arrayed-waveguide grating selects an output optical waveguide in accordance with the wavelength of input light. On the other hand, the star coupler type power splitter distributes input light to individual output optical waveguides independent of wavelength of the light.
As described above, there have been present a wavelength multiplexer having wavelength routing function for selecting an output optical waveguide depending on the wavelength of input light, and an optical power splitter having an optical power splitter function for distributing light to all output optical waveguides independent of the wavelength of input light. Therefore, if an optical integrated circuit having both of wavelength routing function and the optical power splitter function is achieved, such a device can be widely utilized in a PDS test system and other optical communication systems. However, an optical integrated circuit having both functions has not been known in the past.
An optical communication system using an optical fiber and other optical lines uses an optical pulse line test system (OTDR: Optical Time Domain Reflectometer) for detecting a breakage of the optical line and determining the broken position. The optical pulse line test system utilizes that when light transmits in an optical line, a Rayleigh backscatter of the same wavelength generates and transmits in the reverse direction. That is, when an optical pulse (test light) is input to the optical line, Rayleigh backscatter of the same wavelength as the optical pulse continues to generate until the optical pulse reaches the broken point, and output from the input surface of the optical pulse. The broken position in the optical line can be determined by measuring the duration time of the Rayleigh backscatter.
There is an optical branch line (PDS) system as one of optical subscriber systems, construction of which is shown in FIG. 3A. In the optical branch line system, an optical signal transmitting station 10a is provided with an optical signal transmitter 11 and an optical power splitter 12, and N units of optical lines 13 are connected in a star form centering around the optical signal transmitting station 13. Further, an optical power splitter 14a is connected to the tip of each optical line 13, each optical power splitter 14a is connected with M units of optical lines 15 in a star form, and a terminal 16 is connected to the tip of each optical line 15.
In the Figure, only one system is shown after the optical power splitter 14a which is out of the optical signal transmitting station 10a. A system in which the optical power splitter 14a is formed only of passive optical components to ensure high reliability is referred particularly to as an optical branch line system.
Construction of the optical power splitter 14a is shown in FIG. 3B. The optical power splitter 14a combines 4 stages of Y branches to achieve 16 branches. The Y branch has a branch loss of 3 dB in the going way, and light distributed to one of 16-branched optical lines 15 in principle has a branch loss of 12 dB. Further, the Y branch also has a radiation loss of 3 dB in the returning way, and light from one optical line 15 through each Y branch to the optical line 13 is theoretically subject to a 12 dB loss.
The optical branch line system can distribute signal light to a relatively large number of terminals 16 using a fewer number of optical lines. However, test for breakage of the individual lines 15 branched by the optical power splitter 14a is an important problem. That is, a conventional optical line test system using an optical pulse line test device is effective with only a single optical line, and cannot be applied to the optical branch line system. This is because test light is evenly distributed to the individual optical lines 15 branched by the optical power splitter 14a, and it is impossible to determine which optical line is broken.
There has been proposed a method in which, to distinguish the broken line after branching, specific wavelengths are allocated to the individual optical lines, and the optical pulse line test device can change over the wavelength of the test light to select an optical line to be individually tested (F. Yamamoto, I. Sankawa, S. Furukawa, Y. Koyamada, N. Takato, "In-Service Remote Access and Measurement Methods for Passive Double Star Networks", in 5th Conference on Optical Hybrid Access Networks (Montreal 1993), pp. 5.02.01-5.02.06). Construction of the optical branch line test system is shown in FIGS. 4A and 4B. FIG. 4A is a schematic view showing the entire structure, and FIG. 4B shows construction of an optical power splitter 14b. Similar components to those used in the optical branch line system shown in FIGS. 3A and 3B have similar reference symbols.
An optical signal transmitting station 10b comprises an optical pulse line test device (OTDR) 21a, a space switch (SW) 22, and an optical coupler 23 individually provided in the individual optical lines 13 branched by the optical power splitter 12. An optical line 24 for test light is disposed between the optical pulse line test device 21a and the optical power splitter 14b through the space switch 22. The PDS test system makes tests in two steps, a test of the optical line 13 from the optical signal transmitting station 10b to the optical power splitter 14b, and a test of the optical line 15 from the optical power splitter 14b to each terminal 16.
Test of the optical line 13 from the optical signal transmitting station 10b to the optical power splitter 14b is achieved by coupling the test light transmitted from the optical pulse line test device 21a to the optical line 13 by the optical coupler 23. Here, in order to test a plurality of optical lines 13 by a single optical pulse line test device 21a, connection of the optical pulse line test device 21a with the individual optical lines 13 is selected over time using the space switch 22, thereby testing the individual optical lines 13 in time share.
Test of the optical line 15 from the optical power splitter 14b to the individual terminals 16 is achieved by taking in the test light transmitted from the optical pulse line test device 21a to the optical power splitter 14b through the space switch 22 and the optical line 24, thereby testing individually the optical lines 15. That is, test light taken in the optical power splitter 14b is first split into four lines and input to wavelength filters 25.sub.1 to 25.sub.4 which individually pass light of a predetermined wavelength. Here, test light set to a predetermined wavelength is output from the corresponding wavelength filter. On the other hand, signal light input from the optical line 13 is split into 16 lines by repeated halving, and 2-input 2-output 3 dB couplers 26.sub.1 to 26.sub.4 are disposed at a point from four-split to eight-split, where test light output from the individual wavelength filters are multiplexed. This thereby enables testing of a designated optical line corresponding to the wavelength of test light.
Since, in the construction shown in FIG. 4B, test light coupled with the 3 dB coupler 26 is two-split, and further two-split in the next Y branch, four optical lines 15 are tested at a time. This is because selection wavelengths of the wavelength filters 25 cannot be arranged closely. Even though the test light is 16-split to select each wavelength, in the step of coupling with each optical line 15, the test light is subjected to a loss of 15 dB (12 dB by 16-split and 3 dB by the optical coupler), and as a result a loss of Rayleigh backscatter reaches 30 dB inclusive of loss of the test light.
Also, in the construction shown in FIGS. 4A and 4B, test light is finally subject to a 12 dB loss (6 dB by the first 4-split, 3 dB by the 3 dB coupler, and 3 dB by the next Y branch), and the loss of Rayleigh backscatter reaches 24 dB inclusive of the loss of the test light. Further, in practice, there are an optical line loss and a wavelength filter insertion loss, and the total loss is a very high value including these losses.
As described above, the conventional PDS test system splits the test light, and each of the split test light is selected by the wavelength filter to be coupled to the optical line (multiplexed to the split signal light). Thus, the test system is inevitably subject to a substantial loss. Therefore, it has been very difficult to test each optical line 15 after the optical power splitter 14b. Further, when a plurality of optical lines 15 are totally tested, since test light is evenly distributed to the individual optical lines and thus has a very small power, test is very difficult depending on the scale of the branch.